The field of the invention is printing presses, and more particularly, inking systems for printing presses.
An offset printing press typically includes a plate cylinder carrying one or more printing plates. The printing plates have oleophilic surfaces defining an image area, and hydrophilic surfaces defining a non-image area. An inker applies ink to the printing plate which collects on the oleophilic surfaces to form an image which can be transferred to a blanket cylinder which transfers the image to media. By transferring the image from the printing plate onto a blanket roller, and then onto the media, the printing plate does not directly print the image on the media, hence the term offset printing.
The inker applies ink carried on one or more form rollers to the printing plate. When the form roller in the inker engages the printing plate, the ink film on the form roller contacting image areas on the printing plate is split such that approximately one-half of the thickness of the ink film is applied to the image area of the printing plate leaving approximately one-half the ink on the form roller causing a condition referred to as starvation. The ink film on the form roller contacting non-image areas on the printing plate remains on the form roller causing a condition called accumulation.
This combination of accumulation and starvation results in undesirable xe2x80x9cghostedxe2x80x9d images and image repeats being formed on the final printed product. In order to minimize this problem, conventional inkers include a plurality of form rollers which each apply a small amount.
The printed product is monitored to determine when ink density has degraded beyond an acceptable level. In order to control the quality of the printing, conventional printer inkers also include a plurality of adjustable keys to control the amount of ink being applied to the form roller. These keys require constant adjustment to maintain the quality of the printed product.
One attempt to provide a keyless inker incorporated a reverse rotating roller in pressural indentation contact with a main form roller to meter the ink and erase the previous image on the form roller. This prior art inker provided an even film of ink on the printing plate, and inhibited the accumulation and starvation of ink on the form roller. This reverse roller imposed a counter rotating force to the main form roller which increased the power requirements for operating the printing press. In addition the friction caused by the counter-rotating roller generated a tremendous amount of heat that had to be xe2x80x9ctaken awayxe2x80x9d, resulting in more horse power and satellite refrigeration equipment at each printing assembly.
In U.S. Pat. No. 4,453,463, an inker is disclosed for a lithographic printing press in which dampening fluid is applied to a resilient form roller. A blade is mounted to remove the dampening fluid and excess ink directly from the resilient form roller surface. The form roller is rotated into the leading edge of the doctor blade, which is pressure indented to the form roller, and increases the power requirements for rotating the form roller. Furthermore, the blade has a tendency to damage the form roller resilient surface.
U.S. Pat. No. 4,527,479 discloses a method and apparatus for continuously using ink and dampening fluid in a printing system which includes removing ink and dampening fluid from a form roller after the form roller engages the printing plate. Unused printing ink and dampening fluid is removed from the form roller by an idler roller, and a scraping off means scrapes the mixture directly from the idler roller. The mixture is then returned to the reservoir. The ink and dampening fluid removed from the form roller are blended in the reservoir with fresh ink, and recirculated to a distributor line for application to the form roller. This concept works well for a printing press using a low viscosity news print ink which does not dry quickly onto a continuous media. However, for high quality multi colored sheet fed products, the circulation of ink and wash-up requirements is prohibitive.
Another attempt to solve the problem of ghosting is disclosed in U.S. Pat. No. 5,315,930 entitled xe2x80x9cKEYLESS INKING SYSTEM FOR A PRINTING PRESSxe2x80x9d. This patent discloses an inking system for a printing press having an ink injector for supplying ink under pressure, and a device for pumping and metering the ink flow in the injector. The ink injector supplies ink to a fountain roller having an outer brush surface. The fountain roller applies the ink to a pick up roller which transfers the ink through a series of rollers to an applicator roller. The applicator roller has a resilient surface, and applies the ink to two form rollers. A scraper roller engages the applicator roller to remove excess ink therefrom. A scraper blade scrapes ink from the scraper roller. Ink scraped from the scraper roller is transported to an ink reservoir, and is then recirculated using a pump to the ink injector. The inking system in U.S. Pat. No. 5,315,930 has multiple form rollers, and does not provide any means for removing excess ink from the form rollers. In addition, the inking system requires ink recirculation which requires a lengthy wash-up time.
All of the patents referred to above have sought to solve xe2x80x9cghostingxe2x80x9d, starvation, and accumulation problems in inking systems. However, the solutions have complicated the printing press assemblies, require circulating the ink which complicates washing the inker for a color change, and can cause damage to the single form roller.
It is an object of the present invention to provide an inking system which reduces or eliminates xe2x80x9cghostingxe2x80x9d, repeat, starvation and accumulation problems normally associated with prior inking systems.
It is another object of the present invention to provide a relatively uncomplicated inking apparatus which provides uniform inking.
It is another object of the present invention to provide an effective inker having a single form roller for applying a uniform film of ink on a printing plate.
It is another object of the present invention to provide an inking system having effective control of the ink so that it is applied uniformly across the plate two-dimensionally.
It is another object of the present invention to provide an inking system in which wash-up may be efficiently accomplished with minimal use of wash-up fluids or solvents.
It is another object of the present invention to provide an inker that does not require ink circulation to simplify wash-up when changing ink colors.
It is another object of the present invention to provide an inking system having effective control of the ink film applied uniformly across the plate by varying the speed of the ink applicator roller.
It is another object of the present invention to provide an inking system having effective control of the ink being removed from the surface of the form roller.
It is another object of the present invention to provide an inking system in which wash-up may be efficiently accomplished without press assist. The ink applicator and subtractive roll motors provide the inker rotation for wash-up.
These and other objects and features will be apparent from the written description and drawings contained herein.
The invention disclosed herein provides a printing press having a keyless inking system. A conventional key adjusted inking system is an attempt to solve a two dimensional ink distribution problem with a one-dimensional control system (i.e. a row of keys arranged along the width of the press). The present invention controls the two dimensional ink distribution on the surface of a single form roller which inks the printing plate(s).
The inking system of the present invention employs a form roller for applying ink to a printing plate, and a transfer roller adjacent the form roller for removing excess ink from the form roller after printing. A subtractive roller adjacent the transfer roller removes excess ink from the transfer roller, and a scraper blade adjacent the subtractive roller scrapes excess ink from said subtractive roller. An ink reservoir adjacent the scraper blade receives ink scraped from the subtractive roller, and supplies ink for application onto the form roller. An applicator roller adjacent the ink reservoir receives ink from the ink reservoir, and applies the ink to the form roller.
The scraper blade and doctor blade are preferably mounted in a common blade holder which is movable for simultaneously positioning the scraper blade in engagement with the smooth-surfaced ink subtractive roller and the doctor blade in engagement with the surface of the applicator roller. Space between the scraper blade and the doctor blade forms an ink fountain which receives ink from the subtractive roller and applies ink to the applicator roller. Thus, an inker is provided which has an ink reservoir interposed between a subtractive roller which deposits excess ink from the form roller therein, and an applicator roller which receives ink from the ink reservoir for application onto the form roller.
Embodiments of the present invention include a printing system having a rotating plate cylinder carrying a printing plate and a single form roller for applying ink to the printing plate. In accordance with this aspect of the invention the plate cylinder and the form roller are rotated at the same rpm so that the same areas on the form roller contact the same areas on the printing plate during each revolution of the plate cylinder. The plate cylinder and the form roller are configured to have different diameters and, thus, have different surface speeds at a nip formed there between. The system may be equipped with the keyless, subtractive inking system described above. In operation the system is capable of producing an ink film on the two-dimensional surface of the single form roller which essentially eliminates ghosting, repeats, accumulation and starvation. Through simple speed adjustments of the applicator roller, an essentially uniform film of ink may be applied to the image areas of the printing plate. Repeats and ghostings caused by a lack of registration between surfaces of the printing plate and the form roller are eliminated.
In preferred embodiments of the printing system of the present invention, the form roller is of similar size to the plate cylinder. The form roller may be constructed with a removable covering to facilitate maintenance procedures and to reduce the need to remove the relatively large form roller from the press.
In another preferred embodiment of the present invention, the applicator roller has a hard surface formed with an array of wells, adjacent ones of which are interconnected by at least one channel. A doctor blade, which forms part of the ink reservoir, meters ink from the ink reservoir onto the applicator roller. The amount of ink applied to the form roller and then to the printing plate may be adjusted by adjusting the speed of the applicator roller relative to the press speed.
Preferred embodiments of the printing system of the present invention are designed to facilitate efficient and effective ink wash-up. These systems may include mechanisms for disengaging the press drive from the inker and for separately driving the inking system during wash-up. One or more spray bars may be used for applying wash-up fluid to at least one roller in the inking system. In operation the ink subtractive roller may be used to remove a mixture of wash-up fluid and residual ink from the inker system and deposit the mixture into a wash common reservoir during wash-up.
The foregoing is intended to provide a convenient summary of the present disclosure. However, the invention intended to be protected is set forth in the claims hereof.